Fundamentals of Analytical
Chemistry, 10e
Chapter 23: Instruments for Optical
Spectrometry
[Author Name], [Book Title], [#] Edition. © [Insert Year] Cengage. All Rights Reserved. May not be scanned, copied or duplicated, or posted to
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not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
1
Chapter Objectives (1 of 3)
By the end of this chapter, you should know:
•
Sources for spectrochemical measurements.
•
Types of optical materials.
•
How to use laser sources for spectroscopy.
•
How to use monochromators for wavelength selection.
•
The role of interference and interference filters.
•
How to use diffraction gratings.
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2
Chapter Objectives (2 of 3)
• How to use detectors of radiant energy.
• Types of transducers and detectors and their uses.
• The role of the signal-to-noise ratio of spectrometric
measurements.
• How to use phototubes and photomultiplier tubes.
• How to use silicon photodiodes.
• How to use charge-coupled and charge-injection devices.
• How to use thermal detectors.
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3
Chapter Objectives (3 of 3)
• How to use sample containers for UV/visible measurements.
• How to use single- and double-beam spectrophotometers.
• How to use infrared spectrophotometers.
• How to use interferometers for infrared spectrophotometry.
• How to use Fourier transform infrared spectroscopy.
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4
Important Equations
For grating
For a filter
Transducer response
nλ = d(sin i + sin r)
2t
max 
n
G = KP + K′
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5
Introduction
• Because the basic components of analytical instruments for
absorption as well as for emission and fluorescence spectroscopy
are similar in function and in general performance requirements
regardless of whether they are designed for UV, visible, or IR
radiation, they are all frequently called optical instruments.
• Spectroscopy in the UV/visible and IR regions is often called
optical spectroscopy.
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6
23A Instrument Components (1 of 6)
•
Most spectroscopic instruments in the UV/visible and IR regions have
five components:
1. a stable source of radiant energy
2. a wavelength selector
3. one or more sample containers
4. a radiation detector to convert radiant energy to a measurable electrical
signal
5. a signal-processing and readout unit
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23A Instrument Components (2 of 6)
• Figure 23-1 illustrates how these components are configured to
make optical spectroscopic measurements.
• Figure 23-1a shows the arrangement for absorption
measurements.
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23A Instrument Components (3 of 6)
• Figure 23-1b illustrates the configuration for fluorescence
measurements.
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23A Instrument Components (4 of 6)
• Figure 23-1c illustrates the configuration for emission
spectroscopy.
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10
23A Instrument Components (5 of 6)
• An external source of radiation is required for absorption and
fluorescence spectroscopy (Figures 23-1a and b).
• In absorption measurements, the attenuation of the source radiation
at the selected wavelength is measured.
• In fluorescence measurements, the source excites the analyte and
causes the emission of characteristic radiation that is usually
measured perpendicular to the incident source beam.
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11
23A Instrument Components (6 of 6)
• In emission spectroscopy (Figure 23-1c), the sample itself is the
emitter.
 No external radiation source is needed.
 The sample is usually introduced into a plasma or flame that provides
enough thermal energy to cause the analyte to emit characteristic
radiation.
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12
23A-1 Optical Materials
•
The cells, windows, lenses,
mirrors, and wavelength-selecting
elements in an optical
spectroscopic instrument must
transmit or reflect radiation in the
wavelength region being
investigated.
•
Figure 23-2 shows the functional
wavelength ranges for several
optical materials that are used in
the UV, visible, and IR regions of
the spectrum.
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13
23A-2 Spectroscopic Sources (1 of 6)
• For a source to be suitable for spectroscopic studies,
 it must generate a beam of radiation sufficiently powerful for detection
and measurement.
 its output power should be stable for periods of time, which generally
requires a well-regulated power supply.
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14
23A-2 Spectroscopic Sources (2 of 6)
•
•
There are two major types of sources, as
illustrated in Figure 23-3.
1.
Continuum sources emit radiation that
changes in intensity only slowly as a
function of wavelength.
2.
Line sources, which emit a limited number
of spectral lines that each span only a very
narrow range.
A continuum source provides a broad
distribution of wavelengths within a
particular spectral range known as a
spectral continuum.
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15
23A-2 Spectroscopic Sources (3 of 6)
•
Table 23-1 shows examples of continuum sources suitable for various types of
spectroscopy.
Table 23-1 Continuum Sources for Optical Spectroscopy
Source
Wavelength Region, nm
Type of Spectroscopy
Xenon arc lamp
250−600
Molecular fluorescence
H2 and D2 lamps
160−380
UV molecular absorption
Tungsten/halogen lamp
240−2500
UV/visible/near-IR molecular absorption
Tungsten lamp
350−2200
Visible/near-IR molecular absorption
Nernst glower
400−20,000
IR molecular absorption
Nichrome wire
750−20,000
IR molecular absorption
Globar
1200−40,000
IR molecular absorption
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23A-2 Spectroscopic Sources (4 of 6)
• Sources can also be classified as:
 Continuous sources that emit radiation continuously with time.
 Pulsed sources that emit radiation in bursts.
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23A-2 Spectroscopic Sources (5 of 6)
Two examples of continuum sources
with their spectra are tungsten
filament lamps (Figure 23-4; left) and
deuterium lamps (Figure 23-5; right).
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18
Feature 23-1: Laser Sources: The Light
Fantastic (1 of 8)
Lasers have become widely used as sources in certain types of analytical
spectroscopy. To help understand how a laser works, consider an assembly of
atoms or molecules interacting with an electromagnetic wave. For simplicity,
consider the atoms or molecules to have two energy levels: an upper level 2 with
energy E 2 and a lower level 1 with energy E1. If the electromagnetic wave is of a
frequency corresponding to the energy difference between the two levels, excited
species in level 2 can be stimulated to emit radiation of the same frequency and
phase as the original electromagnetic wave. Each stimulated emission
generates a photon while each absorption removes a photon. The number of
photons per second, called the radiant flux φ, changes with distance as the
radiation interacts with the assembly of atoms or molecules.
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Feature 23-1: Laser Sources (2 of 8)
The change in flux, dφ, is proportional to the flux itself; to the difference in the populations
of the levels, n2  n1; and to the path length of the interaction, dz, according to where k is
a proportionality constant related to the absorptivity of the absorbing species.
d   k   n2  n1  dz
If the upper-level population can be made to exceed that of the lower level, there will be a
net gain in flux, and the system will behave as an amplifier. If n2  n1, the atomic or
molecular system is said to be an active medium and to have undergone population
inversion. The resulting amplifier is called a laser, which stands for light amplification by
stimulated emission of radiation.
The optical amplifier can be converted into an oscillator by placing the active medium
inside a resonant cavity made from two mirrors as shown in Figure 23F-1. When the gain
of the active medium equals the losses in the system, laser oscillation begins.
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Feature 23-1: Laser Sources (3 of 8)
Figure 23F-1 Laser cavity. The electromagnetic wave travels back and forth between the mirrors,
and the wave is amplified with each pass. The output mirror is partially transparent to allow only a
fraction of the beam to pass out of the cavity.
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Feature 23-1: Laser Sources (4 of 8)
Population inversion is often achieved by a multilevel atomic or molecular system in
which the excitation process, called pumping, is accomplished by electrical means, by
optical methods, or by chemical reactions. In some cases, the population inversion can be
sustained to produce a continuous wave (CW) output beam that is continuous with
respect to time. In other cases, the lasing action is self-terminating so that the laser is
operated in a pulsed mode to produce a repetitive pulse train or a single shot action.
There are many types of lasers available. The first operating lasers were solid-state
lasers in which the active medium was a ruby crystal. In addition to the ruby laser, there
are many other solid-state lasers. A widely used material contains a small concentration of
Nd3  embedded in a yttrium-aluminum-garnet (YAG) host. The active material is shaped
into a rod and pumped optically by a flashlamp, as illustrated in Figure 23F-2a. The
pump and laser transitions are shown in Figure 23F-2b. The Nd:YAG laser generates
nanosecond pulses with a very high output power at a wavelength of 1.06 μm. The
Nd:YAG laser is popular as a pumping source for tunable dye lasers.
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Feature 23-1: Laser Sources (5 of 8)
Figure 23F-2 Schematic of a Nd:YAG laser (a) and energy levels (b). The pump transitions are in the red
region of the spectrum, and the laser output is in the near infrared. The laser is flashlamp pumped. The
region between the two mirrors is the laser cavity.
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23
Feature 23-1: Laser Sources (6 of 8)
Several other rare earth elements, such as ytterbium, holmium, and erbium, are also used as
dopants in solid-state lasers. Titanium-doped sapphire (Ti:sapphire) is used to produce a tunable
infrared laser. Some versions generate ultrashort pulses of very high output power.
The very common helium-neon (He-Ne) laser is a gas laser that operates in a CW mode. The
He-Ne laser is widely used as an optical alignment aid and as a source for some types of
spectroscopy. The nitrogen laser lases on a transition of the nitrogen molecule at 337.1 nm. It is a
self-terminating pulsed laser that requires a very short electrical pulse for pumping the appropriate
transitions. The N2 laser is also used for pumping tunable dye lasers, as discussed later. Excimer
(excited dimer or trimer) lasers are among the newest gas lasers. Rare-gas halide excimer lasers
were first demonstrated in 1975. In one popular type, a gas mixture of Ar, F2 , and He produces ArF
excimers when subjected to an electrical discharge. The excimer laser is an important UV source
for photochemical studies, for fluorescence applications, and for pumping tunable dye lasers.
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Feature 23-1: Laser Sources (7 of 8)
Dye lasers are liquid lasers containing a fluorescent dye such as one of the
rhodamines, a coumarin, or a fluorescein. These have been made to lase at
wavelengths from the IR to the UV. Lasing typically occurs between the first
excited singlet state and the ground state. The lasers can be pumped by
flashlamps or by another laser such as those discussed previously. Lasing can be
sustained over a continuous range of wavelengths on the order of 40 to 50 nm.
The broad band over which lasing occurs makes the dye laser suitable for tuning
by inserting a grating, a filter, a prism, or an interferometric element into the laser
cavity. Dye lasers are very useful for molecular fluorescence spectroscopy and
many other applications.
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Feature 23-1: Laser Sources (8 of 8)
Semiconductor lasers, also known as diode lasers, obtain population inversion
between the conduction band and the valence band of a pn-junction diode. Various
compositions of the semiconductor material can be used to give different output
wavelengths. Diode lasers can be tuned over small wavelength intervals and can
produce outputs in the IR region of the spectrum. They have become extremely useful in
CD and DVD players, in CD-ROM drives, in laser printers, and in spectroscopic
applications, such as Raman spectroscopy.
Laser radiation is highly directional, spectrally pure, coherent, and highly intense.
These properties have made possible many unique research applications that cannot
easily be achieved with conventional sources. Despite the many advances in laser
science and technology, only recently have lasers become routinely useful in analytical
instruments. Even today, many high-powered or ultrafast lasers can be somewhat difficult
to align, maintain, and use.
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26
23A-2 Spectroscopic Sources (6 of 6)
• The continuum sources for IR radiation are normally heated insert
solids.
• A Globar source consists of a silicon carbide rod that emits IR
radiation when heated to about 1500○C by passing electricity
through it.
• A Nernst glower is a cylinder of zirconium and yttrium oxides that
emits IR radiation when heated to a high temperature by an electric
current.
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27
23A-3 Wavelength Selectors (1 of 17)
•
Spectroscopic instruments in the UV and visible regions are usually
equipped with one or more devices to restrict the radiation being
measured to a narrow band that is being absorbed or emitted by the
analyte.

These devices enhance selectivity and sensitivity of the instrument.

For absorption measurements, narrow bands of radiation diminish the
change for Beer’s law deviations due to polychromatic radiation.
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23A-3 Wavelength Selectors (2 of 17)
•
To restrict the radiation being measured, instruments can use

A monochromator or a filter to isolate the desired wavelength band.

A spectrograph to spread out the wavelengths so they can be detected
with a multichannel detector.
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29
23A-3 Wavelength Selectors (3 of 17)
•
A monochromator uses a grating to disperse a spectrum.

It contains an entrance slit and an exit slit.

The exit slit is used to isolate a small band of wavelengths.

One band at a time is isolated and different bands can be transmitted
sequentially by rotating the grating.

The wavelength ranged passed by this device is the spectral bandpass or
effective bandwidth.
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30
23A-3 Wavelength Selectors (4 of 17)
•
Figure 23-6 shows two types of
monochromators:
(a) grating monochromator.
(b) prism monochromator.
•
Monochromators generally have
diffraction grating to disperse
radiation into component
wavelengths.
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23A-3 Wavelength Selectors (5 of 17)
•
Figure 23-6a shows the path of
radiation through a typical grating
monochromator.
•
Angular dispersion results from
diffraction at the reflective
surface of the refraction grating.
•
The two wavelengths are
focused by the second concave
mirror onto the focal plane,
where they appear as two
images on the entrance slit.
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32
23A-3 Wavelength Selectors (6 of 17)
•
A spectrograph uses a grating to disperse a spectrum.

It contains an entrance slit to define the area of the source to be viewed.

A large opening at its exit allows a range of wavelengths to strike a
multiwavelength detector.
•
When an instrument contains a spectrograph, the sample and wavelength are
reversed from the configuration shown in Figure 23-1a.
•
A spectrograph contains a diffraction grating but has no exit slit, so the
dispersed spectrum impinges on a multiwavelength detector.
•
A polychromator contains multiple exit slits so that several wavelength bands
can be isolated simultaneously.
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23A-3 Wavelength Selectors (7 of 17)
•
The effective bandwidth of a
wavelength selector is the width
of the band of radiation in
wavelength units at half-peak
height.
•
Figure 23-7 shows effective
bandwidth in the exit slit output
as a monochromator is scanned
from λ1  Δλ to λ1  Δλ.
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34
23A-3 Wavelength Selectors (8 of 17)
•
The echellette grating is one of the
most common types of reflection
gratings.
•
Figure 23-8 shows a magnified
cross-sectional view of a few typical
grooves in an echellette-type grating.
•
The grating is grooved or blazed so
that it has relatively broad faces
where reflection occurs and narrow
unused faces to provide highly
efficient diffraction of radiation.
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35
23A-3 Wavelength Selectors (9 of 17)
•
In Figure 23-8, a parallel beam of monochromatic radiation approaches the
grating surface at an angle i relative to the grating normal. The incident beam is
made up of three parallel beams that make up a wave front labeled 1, 2, and 3.
•
The diffracted beam is reflected at the angle r, which depends on the
wavelength of radiation.
•
Equation 23-1 shows the relationship between the angle of reflection r and the
wavelength of the incoming radiation.
nλ = d(sin i + sin r)
(23-1)
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23A-3 Wavelength Selectors (10 of 17)
•
The echellette grating is blazed for use in relatively low orders, but an
echelle grating is used in high orders (>10).
•
The echelle grating is often used with a second dispersive element,
such as a prism, to sort out overlapping orders and provide cross
dispersion.
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37
23A-3 Wavelength Selectors (11 of 17)
•
As shown in Figure 23-9, a
major advantage of a grating
monochromator over a prism
monochromator is that the
dispersion along the focal plane
is, for all practical purposes,
linear.
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23A-3 Wavelength Selectors (12 of 17)
• Reciprocal linear dispersion is the change in wavelength per unit
distance along the focal point of the monochromator.
• The product of reciprocal linear dispersion and slit width is the
spectral or effective bandpass of the monochromator.
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39
23A-3 Wavelength Selectors (13 of 17)
• Gratings can be formed on a concave surface in much the same
way as on a plane surface.
• A concave grating permits the design of a monochromator without
auxiliary collimating and focusing mirrors or lenses because the
concave surface both disperses the radiation and focuses it on the
exit slit.
• Holographic gratings are produced using an optical technique and
are not subject to mechanical errors found in other gratings.
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40
Example 23-1 (1 of 2)
An echellette grating containing 1450 blazes per millimeter was irradiated with a
polychromatic beam at an incident angle 48 degrees to the grating normal. Calculate the
wavelengths of radiation that would appear at angles of reflection of +20, +10, and 0 deg
(angle r, Figure 23-8).
Solution
To obtain d in Equation 23-1, write
d 
1 mm
nm
 10 6
1450 blazes
mm
 689.7
nm
blaze
When r in Figure 23-8 equals +20 deg, λ can be obtained by substituting into Equation 231. Therefore,
689.7 nm
748.4
 
nm
 sin 48  sin 20  
n
n
and the wavelengths for the first-, second-, and third-order reflections are 748, 374, and
249 nm, respectively.
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Example 23-1 (2 of 2)
Similar calculations, shown in the table that follows, reveal that the wavelength in
the second order is one half that in the first order, the wavelength in the third
order is one third that in the first order, and so forth.
Wavelength (nm) for
r, deg
n=1
n=2
n=3
20
748
374
249
10
632
316
211
0
513
256
171
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42
23A-3 Wavelength Selectors (14 of 17)
•
Figure 23-10 shows two
types of filters used in
spectroscopy:
1. Interference filters.
2. Absorption filters.
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43
23A-3 Wavelength Selectors (15 of 17)
•
Interference filters rely on optical
interference to provide a relatively
narrow band of radiation.
•
Figure 23-11 shows the structure of an
interference filter and an example of
conductive interference.
•
The filter contains a thin layer of
transparent dielectric material.
•
A dielectric is a nonconducting
substance or insulator that is usually
optically transparent.
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44
23A-3 Wavelength Selectors (16 of 17)
• Equation 23-2 gives the nominal wavelength for an interference
filter λmax
2t
max 
n
(23-2)
where t is the thickness of the central dielectric layer, η is its
refractive index, and n is an integer called the interference order.
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45
23A-3 Wavelength Selectors (17 of 17)
• Absorption filters are limited to use in the visible region.
• They usually consist of a colored glass plate that absorbs part of
the incident radiation and transmits the desired band of
wavelengths.
• One filter can only isolate a single band of wavelengths, so a new
filter must be used for a different wavelength band.
• In the IR region of the spectrum, an interferometer is used.
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46
23A-4 Detecting and Measuring Radiant
Energy (1 of 27)
• To obtain spectroscopic information, radiant power transmitted,
fluoresced, or emitted must be detected and converted into a
measurable quantity.
• A detector indicates, identifies, or records changes in a physical or
chemical quantity in its environment.
• A transducer converts various types of chemical and physical
quantities into electrical signals.
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47
23A-4 Detecting and Measuring Radiant
Energy (2 of 27)
• The ideal transducer for electromagnetic radiation
 responds rapidly to low levels of radiant energy over a broad
wavelength range.
 produces an electrical signal that is easily amplified with a low
electrical noise level.
 produces an electrical signal linearly related to the radiant power P of
the beam as shown in Equation 23-3.
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48
23A-4 Detecting and Measuring Radiant
Energy (3 of 27)
•
The other variables in Equation 23-3 are

G: the electrical response of the detector in units of current, voltage, or
charge.

K: a proportionality constant that measures sensitivity in terms of electrical
response per unit of radiant power input.

K′: dark current, which is a current produced by some radiation
transducers when no light strikes the device.
G  KP  K 
(23-3)
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49
23A-4 Detecting and Measuring Radiant
Energy (4 of 27)
• Because instruments with a significant dark current response are
usually equipped with an electronic circuit or computer program to
automatically subtract the dark current, Equation 23-3 can usually
be simplified to Equation 23-4.
G = KP
(23-4)
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50
23A-4 Detecting and Measuring Radiant
Energy (5 of 27)
•
Table 23-2 shows common detectors for absorption spectroscopy and divides transducers into
two types: photo detectors and thermal detectors.
Type
Wavelength Range, nm
Photon Detectors
Phototubes
150–1000
Photomultiplier tubes
150–1000
Silicon photodiodes
350–1100
Photoconductive cells
1000–50,000
Thermal Detectors
Thermocouples
600–20,000
Bolometers
600–20,000
Pneumatic cells
600–40,000
Pyroelectric devices
1000–20,000
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51
23A-4 Detecting and Measuring Radiant
Energy (6 of 27)
• Photon detectors are based on the interaction of radiation with a
reactive surface to produce electrons (photoemission) or to
promote electrons to energy states in which they can conduct
electricity (photoconduction).
• Only UV, visible, and near-IR radiation possess enough energy to
cause photoemission to occur.
• Photoconductors can be used in the near-, mid-, and far-IR region
of the spectrum.
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52
23A-4 Detecting and Measuring Radiant
Energy (7 of 27)
•
Two methods are commonly used to detect IR radiation:
1. measuring the temperature rise of a thermally sensitive material.
2. measuring the increase in electrical conductivity of a photoconducting
material.
•
Because the temperature changes involved are tiny, ambient
temperature must be carefully controlled to avoid large errors.
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53
Feature 23-5: Signals, Noise, and the Signalto-Noise Ratio (1 of 3)
The output of an analytical instrument fluctuates in a random way. These
fluctuations limit the precision of the instrument and are the net result of a large
number of uncontrolled random variables in the instrument and in the chemical
system under study. An example of such a variable is the random arrival of
photons at the photocathode of a photomultiplier tube. The term noise is used to
describe these fluctuations, and each uncontrolled variable is a noise source. The
term comes from audio and electronic engineering where undesirable signal
fluctuations appear to the ear as static, or noise. The average value of the output
of an electronic device is called the signal, and the standard deviation of the
signal is a measure of the noise.
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54
Feature 23-5: Signals, Noise, and the Signalto-Noise Ratio (2 of 3)
An important figure of merit for analytical instruments, stereos, compact-disk players,
and many other types of electronic devices is the signal-to-noise ratio (S/N ). The signalto-noise ratio is usually defined as the ratio of the average value of the output signal to
its standard deviation. The signal-to-noise behavior of an absorption spectrophotometer
is illustrated in the spectra of hemoglobin shown in Figure 23F-4. The spectrum at the
bottom of the figure has S/N = 100, and you can easily pick out the absorption maxima at
540 nm and 580 nm. As the S/N degrades to about two in the second spectrum from the
top of the figure, the peaks are barely visible. Somewhere between S/N = 2 and S/N = 1,
the peaks disappear altogether into the noise and are impossible to identify. As modern
instruments have become computerized and controlled by sophisticated electronic
circuits, various methods have been developed to increase the signal-to-noise ratio of
instrument outputs. These methods include analog filtering, lock-in amplification, boxcar
averaging, smoothing, and Fourier transformation.
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55
Feature 23-5: Signals, Noise, and the Signalto-Noise Ratio (3 of 3)
Figure 23F-4 Absorption spectra of
hemoglobin with identical signal levels but
different amounts of noise. Note that the
curves have been offset on the absorbance
axis for clarity.
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56
23A-4 Detecting and Measuring Radiant
Energy (8 of 27)
•
The response of a phototube or
photomultiplier tube results from
the photoelectric effect.
•
Figure 23-12 shows the structure of
a phototube.
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57
23A-4 Detecting and Measuring Radiant
Energy (9 of 27)
•
When a voltage is applied across the electrodes of a phototube, the
emitted photoelectrons are attracted to the positively charged wire
anode and produce a photocurrent.
•
Photoelectrons are electrons that are ejected from a photosensitive
surface by electromagnetic radiation. A photocurrent is the current in an
external circuit that is limited by the rate of ejection of photoelectrons.
•
The photocurrent produced can be amplified and measured.
•
The number of photoelectrons ejected from the photocathode per unit
time is directly proportional to the radiant power of the beam striking the
surface.
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58
23A-4 Detecting and Measuring Radiant
Energy (10 of 27)
• The photomultiplier tube
(PMT) resembles the
phototube but is more
sensitive.
• In place of a single wire
anode, the PMT has a
series of electrodes called
dynodes as shown in
Figure 23-13.
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59
23A-4 Detecting and Measuring Radiant
Energy (11 of 27)
•
With modern electronic instruments, it is possible to detect the electron
pulses resulting from the arrival of individual photons at the
photocathode of a PMT.
•
The pulses are counted, and the accumulated count is a measure of the
intensity of the electromagnetic radiation impinging on the PMT.
•
Photon counting is advantageous when the light intensity, or frequency
of arrival of photons at the photocathode, is low.
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60
23A-4 Detecting and Measuring Radiant
Energy (12 of 27)
• Photoconductive transducers consist of a thin film of a
semiconductor material, deposited often on a nonconducting glass
surface and sealed in an evacuated envelope.
 A semiconductor is a substance having conductivity that lies
between that of a metal and that of a dielectric (an insulator).
 A photoconductor is typically placed in series with a voltage source
and load resistor.
 Voltage drop across the load resistor serves as a measure of the
radiant power of the radiation beam.
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61
23A-4 Detecting and Measuring Radiant
Energy (13 of 27)
•
Crystalline silicon is a semiconductor and a Group IV element.

In a silicon crystal, each of the four valence electrons of silicon is combined
with electrons from four other silicon atoms to form four covalent bonds.

At room temperature, sufficient thermal agitation occurs to liberate an
occasional electron from its bonded state so that it is free to move
throughout the crystal.

Thermal excitation of an electron leaves behind a positively charged hole
that is also mobile.
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62
23A-4 Detecting and Measuring Radiant
Energy (14 of 27)
• Crystalline silicon is a semiconductor and a Group IV element.
 The mechanism of hole movement is stepwise, with a bound electron
from a neighboring silicon atom jumping into the electron-deficient
hole and creating another positive hole in its wake.
 The motion of electrons and holes in opposite directions in
semiconductors is the source of conduction in these devices.
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63
23A-4 Detecting and Measuring Radiant
Energy (15 of 27)
•
Doping enhances the conductivity of silicon.
1. A tiny controlled amount of a Group V or Group III element is distributed
homogeneously throughout a silicon crystal. Valence electrons of the
dopant form covalent bonds with the silicon atoms.
2. Group V elements have four valence electrons that bond with the four
valence electrons of silicon, leaving one electron free to conduct.
3. Group III elements have three valence electrons available to bond with the
valence electrons for silicon, producing an excess of holes.
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64
23A-4 Detecting and Measuring Radiant
Energy (16 of 27)
•
An n-type semiconductor
contains unbonded electrons
(negative charges), as shown
in Figure 23-14 (left).
Electrons are the majority
carrier.
•
A p-type semiconductor
contains an excess of holes
(positive charges), as shown
in Figure 23-15 (right). Holes
are the majority carrier.
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65
23A-4 Detecting and Measuring Radiant
Energy (17 of 27)
• It is possible to fabricate a pn junction or a pn diode, which is
conductive in one direction but not the other.
• Electrical wires are attached to each end of the device.
• Figure 23-16a shows a schematic of a silicon diode.
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66
23A-4 Detecting and Measuring Radiant
Energy (18 of 27)
•
Figure 23-16b shows the junction in
its conduction mode, wherein the
positive terminal of a dc source is
connected to the p region and the
negative terminal to the n region.
•
The diode is forward biased under
these conditions.
•
Bias is a dc voltage, sometimes
called a polarizing voltage, applied
to a circuit element to establish a
reference level for operation.
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67
23A-4 Detecting and Measuring Radiant
Energy (19 of 27)
•
In Figure 23-16b, the excess electrons
in the n region and the positive holes in
the p region move toward the junction,
where they combine and annihilate
each other.
•
The negative terminal of the source
injects new electrons into the n region to
continue the conduction process.
•
The positive terminal extracts electrons
from the p region, creating new holes
that can migrate toward the pn junction.
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68
23A-4 Detecting and Measuring Radiant
Energy (20 of 27)
• Photodiodes are semiconductor pn-junction devices that respond
to incident light by forming electron-hole pairs.
• When voltage is applied to the p diode such that the p-type
semiconductor is negative with respect to the n-type
semiconductor, the diode is said to be reverse biased.
• The majority of electrons are drawn away from the junction, leaving
a nonconductive depletion layer.
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69
23A-4 Detecting and Measuring Radiant
Energy (21 of 27)
• Figure 23-16c shows the
behavior of a silicon junction
that is reverse biased.
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70
23A-4 Detecting and Measuring Radiant
Energy (22 of 27)
•
Diode-array detectors make high-speed spectroscopy possible.
•
One thousand or more silicon photodiodes can be fabricated side by
side on a small silicon chip. Wavelengths can be monitored
simultaneously using diode-array detectors placed along the length of
the focal plane of a monochromator.
•
If the number of light-induced charges per unit time is large compared to
thermally produced charge carriers, the current in an external circuit,
under reverse-bias conditions, is directly related to the incident radiant
power.
•
Added image intensifiers provide gain and allow detection of low light
levels.
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71
23A-4 Detecting and Measuring Radiant
Energy (23 of 27)
• Figure 23-17 is a cross-sectional depiction of one of the pixels in
a charge transfer device.
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72
23A-4 Detecting and Measuring Radiant
Energy (24 of 27)
•
In a charge-injection device (CID) detector, the voltage change arising
from movement of the charge from the region under one electrode to the
region under the other is measured.
•
In a charge-coupled device (CCD) detector, the charge is moved to a
charge-sensing amplifier for measurement.
•
Charge-coupled devices with front-end image intensifiers (ICCDs) can
be gated on and off at selected intervals to provide time resolution for
lifetime studies, for chemical kinetics, or to discriminate against
undesirable signals.
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73
23A-4 Detecting and Measuring Radiant
Energy (25 of 27)
•
Because photons in the infrared (IR) region lack sufficient energy to
cause photoemission of electrons, convenient photon detectors cannot
be used.
•
Currently, most Fourier transform IR spectrometers use a pyroelectric
transducer or a mercury cadmium telluride (MCT) photoconductive
detector.
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74
23A-4 Detecting and Measuring Radiant
Energy (26 of 27)
•
A thermal detector consists of a tiny blackened surface that absorbs IR
radiation and therefore increases in temperature. The temperature rise is
converted to an electrical signal that is amplified and measured.
•
To minimize effects of background radiation, or noise, thermal detectors are
housed in a vacuum and shielded; a rotating slotted disc called a chopper
forces the beam to alternate between maximum and zero intensity.
•
The transducer converts this periodic radiation to an alternating electrical
current that can be amplified and separated from the background dc signal.
•
However, IR measurements are less precise than measurements of UV and
visible radiation.
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75
23A-4 Detecting and Measuring Radiant
Energy (27 of 27)
•
There are four types of thermal transducers used for IR spectroscopy:
1. The thermopile is a tiny thermocouple or group of thermocouples.
2. The bolometer is a conducting element whose electrical resistance
changes as a function of temperature.
3. A pneumatic detector consists of a small cylindrical chamber that is filled
with xenon and contains a blackened membrane to absorb IR radiation and
heat the gas.
4. Pyroelectric detectors are manufactured from crystals of a pyroelectric
material so that a crystal is sandwiched between a pair of electrodes and
produces a temperature-dependent voltage when exposed to IR radiation.
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76
23A-5 Signal Processors and Readout
Devices
•
•
A signal processor is an electronic device that may

amplify the electrical signal from the detector

convert the signal from ac to dc (or the reverse)

change the phase of the signal

filter the signal to remove unwanted components

perform mathematical operations on the signal
Examples of readout devices in modern instruments include digital
meters and computer monitors.
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77
Feature 23-6: Measuring Photocurrents with
Operational Amplifiers (1 of 2)
The current produced by a reverse-biased silicon photodiode is typically 0.1 μA to 100 μA. These
currents, as well as those generated by photomultipliers and phototubes, are so small that they
must be converted to a voltage that is large enough to be measured with a digital voltmeter or
other voltage-measuring device. We can perform such a conversion with the operational amplifier
(op amp) circuit shown in Figure 23F-5. Light striking the reverse-biased photodiode causes a
current I in the circuit. Because the op amp has a very large input resistance, essentially no
current enters the op amp input designated by the minus sign. Thus, current in the photodiode
must pass through the resistor R. The current is conveniently calculated from Ohm’s law: Eout   IR.
Since the current is proportional to the radiant power (P) of the light striking the photodiode, I = kP,
where k is a constant, and therefore, Eout   IR   kPR  k P . A voltmeter is connected to the
output of the op amp to give a direct readout that is proportional to the radiant power of the light
falling on the photodiode. This same circuit can also be used with vacuum photodiodes or
photomultipliers.
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78
Feature 23-6: Measuring Photocurrents with
Operational Amplifiers (2 of 2)
Figure 23F-5 An operational amplifier current to voltage converter used to
monitor the current in a solid-state photodiode.
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79
23A-6 Sample Containers (1 of 3)
•
Sample containers are usually called cells or cuvettes.
•
They must have windows that are transparent in the spectral region of
interest.
•

Quartz or fused silica is required for the UV region and may be used in the
visible region and out to about 3000 nm.

Silicate glass is usually used for the 375 to 2000 nm region.

The most common window region for IR studies is crystalline sodium
chloride.
The best cells have regions that are perpendicular to the direction of the
beam to minimize reflection losses.
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80
23A-6 Sample Containers (2 of 3)
• It is imperative to thoroughly clean cells before and after use
because the transmission characteristics of the cell may be
significantly altered by deposits on the cell walls.
• Matched cells should never be dried by heating in an oven or over
a flame as this may cause physical damage or change the path
length.
• Matched cells should be calibrated against each other regularly
with an absorbing solution.
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81
23A-6 Sample Containers (3 of 3)
•
Figure 23-18 shows typical
examples of cells for the
UV/visible region.
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82
23B Ultraviolet/Visible Photometers and
Spectrophotometers (1 of 2)
• A spectrometer is a spectroscopic instrument that uses a
monochromator or polychromator in conjunction with a transducer
to convert radiant intensities into chemical signals.
• Spectrophotometers are spectrometers that allow measurement of
the ratio of the radiant powers of two beams (required to measure
absorbance).
• Photometers use a filter for wavelength selection in conjunction
with a suitable radiation transducer.
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83
23B Ultraviolet/Visible Photometers and
Spectrophotometers (2 of 2)
• Advantages of specific devices are as follows:
 Spectrophotometers allow the wavelength to be varied continuously,
allowing absorption spectra to be recorded.
 Photometers are simple, rugged, and inexpensive.
• Spectrophotometers usually cover the UV/visible and occasionally
the near-infrared region.
• Photometers are most often used for the visible region.
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84
23B-1 Single-Beam Instruments (1 of 4)
• Spectronic series instruments are commonly used
spectrophotometers.
• The Spectronic 20 is equipped with an occluder, which is a vane
that automatically falls between the beam and the detector
whenever the cylindrical cell is removed from its holder.
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85
23B-1 Single-Beam Instruments (2 of 4)
•
To obtain a percent transmittance reading using the Spectronic 20:
1. First perform the 0% T calibration or adjustment, in which the digital
reading is zeroed out with the sample compartment empty so that the
occluder blocks the beam.
2. Perform the 100% T calibration or adjustment, in which a cell containing
the blank (often the solvent) is inserted into the cell holder and the pointer
is brought to the 100% T mark by adjusting the position of the light control
aperture.
3. Place the sample in the cell compartment and read the percent
transmittance or absorbance from the LCD display.
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86
23B-1 Single-Beam Instruments (3 of 4)
•
The Spectronic 20 has been replaced by the Spectronic 200, which has
features such as a wider spectral range, a spectral bandwidth of 4 nm
instead of 20 nm, and the ability to accommodate square cuvettes.
•
The Spectronic 200 measures 0% T automatically at start-up and can
record 100% T over its entire wavelength range.
•
A spectral scanning mode allows entire spectra to be recorded.
•
An emulation mode provides many of the same operations as legacy
instruments so that methods and procedures developed previously can
still be used.
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87
23B-1 Single-Beam Instruments (4 of 4)
• When using single-beam instruments, the 0% T and 100% T
adjustments should be made immediately before each
transmittance or absorbance measurement.
• To obtain reproducible transmittance measurements, the radiant
power of the source must remain constant during the time that the
100% T adjustment is made and the % T is displayed.
• Single-beam instruments are well-suited for quantitative absorption
measurements at a single wavelength.
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88
23B-2 Double-Beam Instruments (1 of 4)
•
Figure 23-20 compares a single-beam system (a) with two double-beam
designs (b and c).
•
Figure 23-20a shows a single-beam instrument in which radiation from the
filter or monochromator passes through either the reference cell or the
sample cell before striking the photodetector.
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89
23B-2 Double-Beam Instruments (2 of 4)
•
Figure 23-20b shows a double-beam-in-space instrument in which two
beams are formed by a V-shaped mirror called a beam-splitter.
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90
23B-2 Double-Beam Instruments (3 of 4)
Figure 23-20c shows a
double-beam-in-time
instrument in which the beams
are separated in time by a
rotating sector mirror.
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91
23B-2 Double-Beam Instruments (4 of 4)
• Advantages of double-beam instruments are that they
 compensate for all but the most rapid fluctuations in radiant output of
the source.
 compensate for wide variations of source intensity with wavelength.
 are well-suited for continuous recording of absorption spectra.
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92
23B-3 Multichannel Instruments (1 of 3)
• Photodiode arrays and charge-transfer devices are the basis of
multichannel instruments for UV-visible absorption.
• The dispersive system is a grating spectrograph placed after the
sample or reference cell.
• The photodiode array or CCD array is placed in the focal plane of
the spectrograph.
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93
23B-3 Multichannel Instruments (2 of 3)
• With single-beam designs, the array dark current is acquired and
stored in computer memory.
• The raw spectrum of the sample is obtained, and, after dark
current subtraction, the sample values are divided by the source
values at each wavelength to produce the absorption spectrum.
• Multichannel instruments can also be configured as double-beamin-time spectrophotometers.
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94
23B-3 Multichannel Instruments (3 of 3)
•
Figure 23-21 shows the most common, single-beam design.
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95
23C-1 Dispersive Infrared Instruments (1 of 2)
• In most UV/visible instruments, the cell is located between the
monochromator and the detector to avoid photodecomposition of
the sample.
• Photodiode-array instruments avoid this problem because of the
short exposure time of the sample to the beam.
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96
23C-1 Dispersive Infrared Instruments (2 of 2)
•
Older IR instruments were almost always dispersive double-beam
designs, often of the double-beam-in-time variety except that the
location of the cell compartment with respect to the monochromator was
reversed.
•
In IR instruments, the cell compartment is usually located between the
source and the monochromator.

IR is not sufficiently energetic to bring about photodecomposition.

Most samples are good emitters of IR radiation.
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97
23C-2 Fourier Transform Instruments (1 of 4)
• Fourier transform infrared (FTIR) spectrometers have replaced
dispersive instruments in most laboratories.
• FTIR transform spectrophotometers detect all IR wavelengths all
the time. They have greater light-gathering power than dispersive
instruments and consequently better precision.
• The complex calculations required are easily accomplished with
appropriate computers and software.
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98
23C-2 Fourier Transform Instruments (2 of 4)
• FTIR spectrometers contain no dispersing element.
• An interferometer is used to produce interference patterns that
contain IR spectral information (instead of using a
monochromator).
• The interferometer modulates the source signal so it can be
decoded by the mathematical technique of Fourier transformation.
• Most benchtop FTIR spectrophotometers are of the single-beam
type.
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99
23C-2 Fourier Transform Instruments (3 of 4)
•
To collect the spectrum of a sample:
1. The background spectrum is obtained by Fourier transformation of the
interferogram from the background.
2. The sample spectrum is acquired.
3. The ratio of the single-beam spectrum to that of the background spectrum
is calculated and the absorbance or transmittance versus wavelength or
wavenumber are plotted.
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100
23C-2 Fourier Transform Instruments (4 of 4)
•
Major advantages of FTIR instruments over dispersive
spectrophotometers include

better speed and sensitivity

better light-gathering power

more accurate wavelength calibration

simpler mechanical design

virtual elimination of any contribution from stray light and IR emission
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101
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (1 of 14)
Fourier transform infrared (FTIR) spectrometers utilize an ingenious device called a Michelson
interferometer, which was developed many years ago by A. A. Michelson for making precise
measurements of the wavelengths of electromagnetic radiation and for making incredibly accurate
distance measurements. The principles of interferometry are utilized in many areas of science
including chemistry, physics, astronomy, and metrology and are applicable in many regions of the
electromagnetic spectrum.
A diagram of a Michelson interferometer is shown in Figure 23F-6. It consists of a collimated
light source, shown on the left of the diagram; a stationary mirror at the top; a moveable mirror at
the right; a beam-splitter; and a detector. The light source may be a continuum source as in FTIR
spectroscopy, or it may be a monochromatic source such as a laser or a sodium arc lamp for other
uses such as, for example, measuring distances. The mirrors are precision-polished ultraflat glass
with a reflective coating vapor deposited on their front surfaces. The moveable mirror is usually
mounted on a very precise linear bearing that allows it to move along the direction of the light
beam while remaining perpendicular to it as shown in the diagram.
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102
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (2 of 14)
Figure 23F-6 Diagram of a Michelson interferometer.
A beam from the light source on left is split into two
beams by the beam-splitter. The two beams travel two
separate paths and converge on the detector. The two
beams A′ and B converge in the same region of space
and form an interference pattern. As the movable
mirror on the right is moved, the interference pattern
shifts across the detector and modulates the optical
signal. The resulting reference interferogram is
recorded and used as a measure of the power of the
incident beam at all wavelengths. An absorbing
sample is then inserted into the beam, and a sample
interferogram is recorded. The two interferograms are
then used to compute the absorption spectrum of the
sample.
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103
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (3 of 14)
The key to the operation of the interferometer is the beamsplitter, which is usually a partially
silvered mirror similar to the “two-way” mirrors often seen in retail stores and police interrogation
rooms. The beam-splitter allows a fraction of the light falling on it to pass through the mirror, and
another fraction is reflected. This device works in both directions so that light falling on either side
of the beam-splitter is partially reflected and partially transmitted.
For simplicity, we will use as our light source the blue line of an argon-ion laser. Beam A from
the source impinges on the beam-splitter, which is tilted at 45° to the incoming beam. Our beamsplitter is coated on the right side, so Beam A enters the glass and is partially reflected off the back
side of the coating. It emerges from the beam-splitter as Beam A′ and moves up toward the
stationary mirror where it is reflected back down toward the beam-splitter. Part of the beam is then
transmitted down through the beam-splitter toward the detector. Although the beam loses some
intensity with each interaction with the stationary mirror and the beam-splitter, the net effect is that
a fraction (Beam A′) of incident Beam A ends up at the detector.
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Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (4 of 14)
In its first interaction with the beam-splitter, the fraction of Beam A that is transmitted emerges to the right
toward the moveable mirror as Beam B. It then is reflected back to the left to the beam-splitter where it is
reflected down toward the detector. With careful alignment, both Beam A′ and Beam B (shown separately in
the diagram for clarity) are collinear and impinge on the detector at the same spot.
The overall purpose of the interferometer optics is to split the incident beam into two beams that move
through space along separate paths and then recombine at the detector. It is in this region that the two beams,
or wavefronts, interact to form an interference pattern. The origin of the interference pattern is illustrated in
Figure 23F-7, which is a two-dimensional representation of the interaction of the two spherical wavefronts.
Beam A′ and Beam B converge and interact as two point sources of light represented in the upper portion of
the figure. When the two beams interfere, they form a pattern similar to the one shown. In regions where the
waves interfere constructively, bright bands appear, and where destructive interference occurs, dark bands
form. The alternating light and dark bands are called interference fringes. These fringes appear at the
detector as the output image shown at the bottom of the figure. In the earliest versions of the Michelson
interferometer, the detector was the human eye aided by a telescope. The fringes could be counted or
measured through the telescope.
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105
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (5 of 14)
Figure 23F-7 A two-dimensional representation of the
interference of two monochromatic wavefronts of the
same frequency. Beam A′ and Beam B at the top form
the interference pattern in the middle, and the two
wavefronts constructively and destructively interfere.
The image shown at the bottom would appear at the
output of the Michelson interferometer perpendicular to
the plane of the two-dimensional interference pattern.
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106
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (6 of 14)
When the moveable mirror is moved to the left at constant velocity, the interference pattern gradually
sweeps past the detector as the path that Beam B follows is gradually shortened. The form of the interference
pattern remains the same, but the positions of constructive and destructive interference are shifted as the path
difference changes. For example, if the wavelength of our laser source is λ, as we move the mirror a distance
of λ 4, the path difference between the two beams changes by λ 2, and where there was constructive
interference, now there is destructive interference. If we move the mirror another λ 4, the path difference
changes again by λ 2, and we again return to constructive interference. As the mirror moves, the two
wavefronts are shifted in space relative to one another, and alternate light and dark fringes sweep across the
detector, as illustrated in Figure 23F-8a. At the detector, find the sinusoidal intensity profile shown in Figure
23F-8b. This profile is called an interferogram. The net effect of the constant uniform motion of the mirror is
that the light intensity at the output of the interferometer is modulated, or systematically varied, in a precisely
controlled way as shown in the figure. In practice, it turns out not to be very easy to move the interferometer
mirror at a constant, precisely controlled velocity. There is a better and much more precise way to monitor the
mirror motion using a second parallel interferometer. In this example, just assume that we can measure and/or
monitor the progress of the mirror and compensate for any nonuniform motion computationally.
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107
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (7 of 14)
Figure 23F-8 Formation of interferograms at the output of the
Michelson interferometer. (a) Interference pattern at the
output of the interferometer resulting from a monochromatic
source. (b) Sinusoidally varying signal produced at the
detector by the pattern in (a). (c) Frequency spectrum of the
monochromatic light source resulting from the Fourier
transformation of the signal in (b). (d) Interference pattern at
the output of the interferometer resulting from a two-color
source. (e) Complex signal produced by the interference
pattern of (d) as it falls on the detector. (f) Frequency
spectrum of the two-color source.
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108
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (8 of 14)
We have established that a Michelson interferometer with a monochromatic light
source produces a sinusoidally varying signal at the detector when the mirror is moved at
constant velocity. Now, we must investigate what happens to the signal once it is
recorded. Although the characteristics of Michelson interferometers have been well known
for over a century and the mathematical apparatus for dealing with the data has been in
place for nearly two centuries, the device could not be used routinely for spectroscopy
until two developments occurred. First, high-speed, inexpensive computers had to
become available. Second, appropriate computational methods had to be invented to
handle the huge number of rather routine calculations that must be applied to the raw data
acquired in interferometric experiments. Briefly, the principles of Fourier synthesis and
analysis tell us that any waveform can be represented as a series of sinusoidal
waveforms, and correspondingly, any combination of sinusoidal waveforms can be broken
down into a series of sinusoids of known frequency. Apply this idea to the sinusoidal signal
detected at the output of the Michelson interferometer shown in Figure 23F-8b.
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109
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (9 of 14)
If we subject the signal in the figure to Fourier analysis via a computer algorithm called
the fast Fourier transform (FFT), we obtain the frequency spectrum illustrated in Figure
23F-8c. Notice that the original waveform in Figure 23F-8b is a time-dependent signal; the
resultant output from the FFT is a frequency-dependent signal. In other words, the FFT
takes amplitude signals in the time domain and converts them to power in the frequency
domain. Since the output of the interferometer is a sine wave of a single frequency, the
frequency spectrum shows a single spike of frequency v, the frequency of the original sine
wave. This frequency is proportional to the optical frequency emitted by the laser source but
of much lower value so that it can be measured and manipulated electronically. In many
instruments, the interferometer is modified to obtain a second sine wave at the output. One
way to do this is to simply add a second wavelength to our light source. Experimentally, a
second laser or another monochromatic light source at the input of the interferometer gives
us a beam that contains just two wavelengths.
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Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (10 of 14)
For example, assume that the second wavelength is one quarter of the first one
so that the second frequency is 4v. Further assume that its intensity is one half the
intensity of the original source. As a result, the signal appearing at the output of the
interferometer exhibits a pattern somewhat more complex than in the singlewavelength example, as shown in Figure 23F-8d. The detector signal plot appears
as the sum of two sine waves as depicted in Figure 23F-8e. Then apply the FFT to
the complex sinusoidal signal to produce the frequency spectrum of Figure 23F-8f.
This spectrum reveals just two frequencies at v and 4v, and the relative magnitudes
of the two frequency spikes are proportional to the amplitudes of the two sine
waves composing the original signal. The two frequencies correspond to the two
wavelengths in our interferometer light source, and the FFT has revealed the
intensities of the source at those two wavelengths.
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Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (11 of 14)
To illustrate how the Michelson interferometer is used in practical experiments,
place a continuum infrared light source (see Figure 23F-9a) containing a huge
number of wavelengths at the input of the interferometer. As the mirror moves
along its path, all wavelengths are modulated simultaneously, producing the very
interesting interferogram shown in Figure 23F-9b. This interferogram contains all
the information required in a spectroscopy experiment regarding the intensity of
the light source at all its component wavelengths.
As suggested in the previous section, there are a number of advantages to
acquiring intensity information in this way rather than using a scanning
spectrometer. First, there is the advantage of speed. The mirror can be moved in a
matter of seconds, and a computer attached to the detector can collect all
necessary data during the course of the mirror scan.
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Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (12 of 14)
Figure 23F-9 (a) Spectrum of a continuum
light source. (b) Interferogram of the light
source in (a) produced at the output of the
Michelson interferometer.
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113
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (13 of 14)
In just a few more seconds, the computer can perform the FFT and produce the
frequency spectrum containing all the intensity information. Next is Fellgett’s
advantage, which suggests that Michelson interferometers are capable of
producing higher signal-to-noise ratios in shorter time than equivalent dispersive
spectrometers. Finally, we have the throughput, or Jacquinot’s advantage, which
permits 10 to 200 times more radiation to pass through a sample compared to
standard dispersive spectrometers, which are limited by entrance and exit slits.
These advantages are often partially offset by the lower sensitivity of detectors that
are used in FTIR spectrometers. Under these circumstances, the speed of the
measurement process and the simplicity and reliability of FTIR spectrometers
become primary considerations.
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114
Feature 23-7: How Does a Fourier Transform
Infrared Spectrophotometer Work? (14 of 14)
Up to this point in our discussion of the FTIR spectrometer, we have shown how the Michelson
interferometer can provide intensity information for a light source as a function of wavelength. To
collect the IR spectrum of a sample, we must first obtain a reference interferogram of the source
with no sample in the light path, as shown in Figure 23F-6. Then, the sample is placed in the path
as indicated by the arrow and dashed box in the figure, and once again, we scan the mirror and
acquire a second interferogram. In FTIR spectrometry, the sample absorbs infrared radiation, which
attenuates the beams in the interferometer. The difference between the second (sample)
interferogram and the reference interferogram is then computed. Since the difference in
interferogram depends only on the absorption of radiation by the sample, the FFT is performed on
the resulting data, which produces the IR spectrum of the sample. We discuss a specific example
of this process in Chapter 24. Finally, note that the FFT can be accomplished using the most basic
modern personal computer equipped with the appropriate software. Many software packages such
as Mathcad, Mathematica, Matlab, and even the Data Analysis Toolpak of Microsoft Excel have
Fourier analysis functions built in. These tools are widely used in science and engineering for a
broad range of signal-processing tasks.
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115
Analytical Chemistry Online Activity
Search for companies that manufacture monochromators. Navigate to
websites of these companies, and find a UV/visible monochromator of the
Czerny-Turner design that has better than 0.1 nm resolution. List several
other important specifications of monochromators, and describe what they
mean and how they affect the quality of analytical spectroscopic
measurements. From the specifications and, if available, prices, determine
the factors that have the most significant effect on the cost of the
monochromators.
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116
Key Terms Activity (1 of 2)
•
Angular dispersion
•
Echellete grating
•
Charge-transfer device
•
Effective bandwidth
•
Continuum source
•
Excimer laser
•
Dark current
•
Fellgett’s advantage
•
Detector
•
Focal plane
•
Diode array
•
Gas laser
•
Dynode
•
Holographic grating
•
Echelle grating
•
Interference filter
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117
Key Terms Activity (2 of 2)
•
Interferogram
•
Reciprocal linear dispersion
•
Jacquinot’s advantage
•
Singlet state
•
Michelson interferometer
•
Spectral bandpass
•
Monochromator
•
Spectrograph
•
Nernst glower
•
Thermopile
•
Photocurrent
•
Transducer
•
Polychromator
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118
Assessments: Discussion Questions (1 of 2)
23-1. Describe the differences between the following pairs of terms, and list
any particular advantages possessed by one over the other:
a) solid-state photodiodes and phototubes as detectors for electromagnetic
radiation.
b) phototubes and photomultiplier tubes.
c) filters and monochromators as wavelength selectors.
d) conventional and diode-array spectrophotometers.
22-3. Why do quantitative and qualitative analyses often require different
monochromator slit widths?
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119
Assessments: Discussion Questions (2 of 2)
23-4. Why are photomultiplier tubes unsuited for the detection of
infrared radiation?
23-5. Why is iodine sometimes introduced into a tungsten lamp?
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120
Challenge Problem (1 of 4)
23-24. Challenge Problem: Horlick has described principles of the
Fourier transform, interpreted them graphically, and described how
they may be used in analytical spectroscopy. Read the article and
answer the following questions.
(a) Define time domain and frequency domain.
(b) Write the equations for the Fourier integral and its transformation,
and define each of the terms in the equation.
G. Horlick, Anal. Chem., 1971, 43(8), 61A-66A, DOI: 10.1021/ac60303a029.
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121
Challenge Problem (2 of 4)
(c) The paper shows the time-domain signals for a 32-cycle cosine wave, a 21cycle cosine wave, and a 10-cycle cosine wave as well as the Fourier
transforms of these signals. How does the shape of the frequency-domain
signal change as the number of cycles in the original waveform changes?
(d) The author describes the phenomenon of damping. What effect does damping
have on the original cosine waves? What effect does it have on the resulting
Fourier transformations?
(e) What is a resolution function?
(f) What is the process of convolution?
(g) Discuss how the choice of the resolution function can affect the appearance of
a spectrum.
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122
Challenge Problem (3 of 4)
(h) Convolution may be used to
decrease the amount of noise in
a noisy spectrum. Consider the
following plots of time-domain
and frequency domain signals.
Label the axes of the five plots.
For example, (b) should be
labeled as amplitude versus time.
Characterize each plot as either
a time-domain or a frequencydomain signal.
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not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
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Challenge Problem (4 of 4)
(i) Describe the mathematical relationships among the plots. For
example, how could you arrive at (a) from (d) and (e)?
(j) Discuss the practical importance of being able to reduce noise in
spectroscopic signals.
Skoog, West, Holler, and Crouch, Fundamentals of Analytical Chemistry, 10e. © 2022 Cengage. All Rights Reserved. May
not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
124